Ligno-cellulose biomass comprises water, volatile compounds and fixed carbon. The roasting of the biomass allows evaporation of the water and of a portion of the volatile compounds, in order to preserve the most energy-containing compounds.
The roasting of biomass is for example used for producing a fuel having a higher energy/weight ratio and millability than those of the initial biomass and which has better hydrophobic characteristics.
Roasting consists of heating the biomass to a high temperature, generally comprised between 200° C. and 350° C., in a low-oxygen atmosphere, in order to avoid any combustion.
Roasting is for example applied by exposing the biomass to a flow of hot gases produced by a burner integrated into the gas loop supplying the roasting column and burning the roasting gases by means of the addition of an additional fuel in the presence of air used as an oxidizer. WO2007/000548 and WO2011/086262 disclose such methods for roasting biomass.
This type of configuration has the necessity of controlling the dioxygen level in the combustion gases at the outlet of the burner and therefore at the inlet of the roasting column. The prior art mentions a maximum threshold of 2% of oxygen in the flow of hot gases, or, in practice, this value is extremely difficult to maintain permanently during continuous utilization. This threshold was set so as to prevent any undesirable combustion of the biomass during roasting.